Publications and other reference materials referred to herein are numerically referenced in the following text and respectively grouped in the appended Bibliography which immediately precedes the claims.
Achromatic wave plates are key components in many instruments and optical systems particularly in polarization control. Due to the importance of these components many investigators have presented various designs for achromatic quarter wave plates (AQWP) and achromatic half wave plates (AHWP) based on various optical principles [1-11].
A cascade of two or more crystalline wave plates is a popular approach to design achromatic wave plates, while the wave plates in the cascade can be produced from the same or different materials with specific thickness and optical axis orientation of each one. The design of an achromatic wave plate in the visible range (using mica) formed from a combination of three retarders of the same material was proposed by Pancharatnam [1], in which the first and the third retarder are identical (thickness and orientation) while the one between has a specific thickness and orientation. For the terahertz region, a design composed of six retarders was presented in ref [2]. Saha et al [3,4,5] presented different AQWP designs using crystalline quartz plates. The first design is a combination of three plates that can operate as AQWP and AHWP in the near infrared region by changing the orientation of the middle plate; the other two designs present an AQWP in the 500-700 nm region while one of them composed from four plates and the last from two plates.
Two twisted nematic liquid crystal retarders were also used to demonstrate an AQWP [6] in the visible range and compared with the three element Pancharatnam's type achromatic design. A design of AQWP was proposed using subwavelength grating structures (SWG) based on their form birefringence [7]. The first physical implementation of the form birefringence of gratings as quarter wave plate was reported for the mid-infrared region [8]. Later an optimization for angle-insensitive design based on SWG was described [9] and an IR dual band AQWP (MWIR and LWIR) designed from a combination of four SWG's [10]]. The main limitation in this concept is the difficult fabrication especially in the visible and the IR region. The total internal reflection in the surface of Si—SiO2 was used also to design an AQWP in the near and mid-infrared regions [11] while the limitation on this approach is the voluminous system comparatively.
It is a purpose of the present invention to provide a new design of achromatic wave plates based on two or more retarders made of electrooptic or magnetooptic materials.
Further purposes and advantages of this invention will appear as the description proceeds.